Jump to content
UltravioletPhotography

Bowens Gemini 500R emission spectrum


enricosavazzi

Recommended Posts

enricosavazzi

I am playing with a new spectroscope ( https://www.lasertac...eter-200-1200nm ). Bits and pieces are still missing, for example I cannot yet use the spectroscope´s external triggering input, and was forced to take an exposure of 1s and trigger the flash manually. No doubt this added quite some background noise, but the results should be valid except in the smallest details because the spectra are highly repeatable. This spectrometer is built around a linear CCD sensor and the exposure time is adjustable in a large interval, so it easily records flash exposures, and the problems with long-exposure noise are similar to those with a digital camera. I am simply trusting that the spectroscope is correctly aligned and calibrated at the factory and shows the right wavelengths and intensity scale, since I have no practical way to calibrate it myself.

 

I was forced to mount a cosine corrector that came with the spectroscope (a ferrule containing a small teflon disk of unknown thickness) at the end of the optical fiber to prevent the flash from saturating the spectroscope even from quite far away. This may have introduced some UV absorption, but the bias in the 300-400 nm range should not be very large.

 

The Bowens Gemini 500R is a studio flash with a specified power emission of 500 Ws (and unlike cheap studio flash units, does seem to emit this power). I replaced the gold-sputtered flash tube with an uncoated one (original Bowens replacement). Since this uncoated tube is designed for flash photography, it may still have a UV-cutting coating or glass material (especially to block UVB and UVC). In the UVA range it emits a lot more than the coated tube, and allows even the use of 320 nm narrow bandpass filters.

 

I recorded the spectra at full power and minimum power (-5 stops), exported the data to Excel and normalized it to the maximum peak value of each spectrum. Of course there would be other, perhaps better, ways to normalize the data for comparing the spectra. The most interesting thing is that the emission grows and then peters out quite gradually, but is also superposed with a few high, very narrow peaks in the VIS and NIR (not in the NUV). The relative height of these peaks increases when the power is decreased. So, if one wants a more uniform spectrum, one should use the flash at high power. Another thing to note is that the NIR emission (more than enough for NIR photography) is concentrated in a few peaks.

 

I have not checked yet if these peaks correspond to Xe emission lines, which would make it possible to verify the correctness of the X axis. I suspect so. This particular spectrometer configuration (there are several to choose from - but you have to choose one when ordering, and the factory customizes it accordingly) should give a 2 nm overall precision and a sub-nm resolution, and a 200-1,200 nm continuous range.

 

This behavior is not necessarily the same in all flash units, especially the small battery-powered ones. This Bowens adjusts the power output in a complex way, by switching in and out multiple capacitors as well as changing their charge voltage. Small flash units usually reduce the emitted power by quenching the discharge with a thyristor, thus making it shorter in time. This may affect UV emission among other things, since, as far as I remember, the initial part of the discharge is more UV-rich than the rest.

red=full power

blue=min power

post-60-0-17302400-1487871901.jpg

Link to comment
enricosavazzi

Can you calibrate it with something of known wavelength, such as a mercury lamp or fluorescent tube?

It is calibrated at the factory (a large part of the cost of these devices is due to factory alignment and calibration, in fact), and from what I have seen the calibration seems precise. For example, my MTE 301 emission peak reads at 367.something nm, which is well within specifications for the Nichia LED.

I have a few fluorescent tubes to try as well.

Link to comment
enricosavazzi

An interesting comparison with the Metz 52 AF-1, a battery-powered flash. Its UV-filtering is obviously very effective. Some of the strongest peaks correspond to those in the Bowens studio flash above.

blue=full power

red=minimum power (1/128)

post-60-0-95354300-1488016809.jpg

Link to comment
Bill De Jager

Thanks for doing these tests, Enrico. I have a couple of Bowens Gemini 750 units I bought used at a good price but I haven't used them yet. I realized belatedly that the UV emissions would not be friendly to interior furnishings, so they await creation of a studio in the garage which is currently a low priority. Given my sunny climate I can usually rely on that source of UV light.

 

One thing I tried looking into is blocking some of the extraneous wavelengths to reduce damage to sensitive subjects such as flowers when doing UV photography. My concept was to filter light progressively, taking out first the near IR, then the visible, to reduce heating of the filters. It turned out that large filter glass is expensive and IR filter glass may not pass much UV, depending on the type. Now it looks like it might be better to take out the visible light in two increments with shortpass filters, if the cost isn't too high.

 

B&H currently has several used (demo condition) Bowens Gemini units of varying power output for sale. Apparently the Gemini series of monolights has been superseded by a newer one.

Link to comment
  • 2 weeks later...

Enrico, I have been reading about UV lighting, trying to get more light for my UV photos and I don't understand why some light sources recommend filtering the light and for others it's all about making sure there is no filter cutting out UV from the visible.

 

For example, why would you need to add a UV filter to a torch (flashlight) or fluorescent light with filters like the Rosco UV Pass filter (Permacolor #3660 Double Coated UV Pass) or a filter stack for a converted Canon Speedlite 199A or Vivitar 285 (Hoya U-340 + Schott S8612) to make it a UV Flash. Still another example is the one I experimented with, the Broncolor UV attachment that goes in front of a Broncolor flash tube.

 

On the other side there are these Bowens monolights with a Clear Flash tube instead of the UV Coated version, the Broncolor Minicom 80 heads mentioned numerous times on the forum and other monolights including the Alien Bees and White Lightning that produce lots of UV light without filters.

 

I guess what I'm asking is, can you add UV to a light source by adding a filter or is it fixed be the nature of the source and making sure you don't limit the transmission with a UV Coated bulb or pyrex dome?

 

Any feedback would be a huge help. Thanks, Michel

Link to comment

Enrico, Really cool spectrometer. I want one :-)

 

Michel, You don't need to filter your light if you are shooting with a UV-only filter. If you are shooting visual florescence, which some people often call 'ultraviolet',

which is better called UV induced visual fluorescence (UVIVF), then you are shooting visual, and your light course, no matter how narrow 365nm,

could/should be filtered to not transmit any possible light above the UV range.

Sometimes even with a narrow band 365nm light source you will get some bit of visible blue light that leaks, and can be seen by the camera.

For UVIVF you only want UV light, nothing above 400nm, and you also want to filter the lens of the camera when shooting UVIVF with a yellow longpass filter,

to prevent the camera from seeing any reflected UV below 400nm.

You want the camera to only see 400/420nm and above, and of course not IR either.

 

If you are shooting UV only, then you only need a UV capable lens, and a UV-only filter (or stack), and a light source that contains UV-A, such as outdoor light does naturally,

or whatever other light you decide contains enough UV.

Most UV torches are narrow band 365nm, which work great for UVIVF, but are mostly monochrome for UV-only, so if you want the false colors of UV-A, and not a monochrome look,

then you want to use a light source other than a UV torch.

 

The light either contains UV, or it doesn't. Any filter you use for a light only blocks some part (or all) of the spectrum of that light source. So yes, the UV of a light is 'fixed by the nature of the source'.

 

As far as that Rosco filter you linked above.

I would like to test one, but did you read the one user review that was posted?

"Also passes violet which made it impractical for my purpose as I wanted to block all the visual.

Maybe OK if passing violet OK."

If the filter transmits out of the UV-A band 320nm to 400nm, then it may not truly be a UV-only filter, and your camera will see visual or even possibly IR also.

Here is a transmission graph of that filter, you will see that it has a bit of a leak not only in the 400nm range, but also in the 700nm range, these look small, but especially 700nm will muddy the waters.

http://cdn3.bigcomme...80.1280.jpg?c=2

 

I should ask, what are your photographing? UV-only? Or UVIVF (black light photography)?

Link to comment

For digital cameras, it is important to realise the Broncolor UV Attachment outputs a very healthy amount of IR besides some strongly filtered UV. Thus one always needs some kind of filter or stack to allow UV through and block IR. This additional filtration preferably is to be done over the lens.

 

The Broncolor gizmo likely was developed in the film era in which IR sensitivity was a non-issue. Never use it with a digital camera without extra filtration as described above.

Link to comment

This is super helpful. Thanks for helping me make sense of it all.

 

I'm trying to get more UV and I actually own some of the Bowens Gemini 500R lights. I will get the clear bulbs and see if that makes a difference. Currently the flash tubes are UV coated and barely put out any UV light. I'll try the Bowens BW-2030 Two-Pin Clear Flashtube for Gemini 500R and let you know if they put out more UV.

 

Cadmium, I am photographing people.

Link to comment

Michel, let me make some general remarks about filtration.

 

Both the lens and the illumination can be filtered. We filter a lens to make sure only the desired wavelength is recorded on a converted sensor. We filter the light source to make sure only the desired wavelength is output by the illumination device.

 

In some (perhaps most?) shooting situations both the lens and the light source must be filtered. For example when shooting visible fluorescence induced by UV light ("black light"), we must filter the lens with a Visible-pass filter and filter the illumination with a UV-pass filter. This is because many UV light sources output some visible light too, in the violet range usually.

 

In other situations you can get away with using only a lens filter. For example when shooting reflected UV scenes in sunlight, all we can do is to place a tight UV-pass filter on the lens. When shooting reflected UV scenes with a simple UV-flash (non-studio type), it is usually also OK to use only a good UV-pass filter on the lens.

 

If the illumination source is really strong -- like with a Broncolor -- as Bjørn has noted, it is best to filter both light and lens.

 

All filters (almost all?) can be forced into passing unwanted wavelengths if a really strong illumination source is used for a long enough exposure. I usually test forced light at around 30" (maximum exposure length without going to bulb). If filtration prevents most of the unwanted wavelengths for a 30" exposure, then I'm pretty sure to get what I want using a more typical exposure.

Link to comment

And....

 

A bit of visible violet leakage just past 400nm in a UV photo does little harm. But longer visible wavelengths might begin to contaminate the photo. Infrared leakage is deadly to reflected UV photography causing a wash-out of UV-dark areas.

 

You might have noticed yourself how IR can contaminate visible photos. Certain synthetic blacks can appear purplish, pinkish or washed out in visible light photos if IR is leaking through. Leica had a little problem with that in their original digital effort -- the M8 -- resulting in a hasty distribution of free IR-blockiing filters to M8 owners. Subsequent digital Ms had a much stronger internal UVIR-blocking filter.

Link to comment
Andy Perrin
Heh. UV photography has a steep learning curve! You find out all the things you took for granted in visible light photography, like lenses that adequately transmit and focus light, can no longer be taken for granted.
Link to comment
Yes Andy. UV is a world of it's own. I've been a commercial photographer for 15 years and almost nothing I understand about cameras, lights or photography seems to apply in this arena. It's tricky stuff but I think I'm starting to get it.
Link to comment
Exposure is still exposure in UV or in IR. You just need to get the filters, lens and light right. "-)
Link to comment

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
×
×
  • Create New...